Strain analysis for early leakage detection and geomechanical monitoring at CO2 storage sites using distributed fiber optic strain sensing

Abstract

When CO₂ is injected into deep saline aquifers, the resulting pressure build-up may cause microseismicity, fault reactivation, and induce damaging earthquakes. Continuous strain data are needed to measure vertical strain migration. Deploying a fiber-optic cable behind a well casing for subsurface geomechanical monitoring offers the opportunity to continuously track the deformation (strain) along the fiber-optic cable. In this study, we conducted three water injection field tests and measured the strain profiles by using distributed fiber optic strain sensing (DFOSS) technique. The first water injection test results showed that the two fiber cable strain measurements at approximately the same distance from the injection well had a strain sensitivity difference of approximately 10 με. The strain sensitivities from wells with different monitoring distances were almost the same. Strain profiles from the other two injection tests showed that the strain responses reveal alternation of sand/silt and reservoir heterogeneity, despite the distances from the injector. In the second injection test, the delayed strain response appeared 10 m above the injection zone associated with the pressure breakdown. In the third test, the DFOSS showed high strain sensitivity with increasing injection pressure, and it could capture a leakage that occurred in the upper zone. The high strain sensitivity of the armored cable, which showed almost the same sensitivity as the flat cable, provides insights into the cable design for deep-well applications. Thus, our results demonstrate the high potential of using DFOSS for caprock and wellbore integrity, leakage monitoring, and geomechanical modeling at geological CO₂ storage sites.